The invention relates to systems and methods of track changing an optical signal beam in response to the frequency selective phase characteristics of a resonator.
In electromagnetic devices, from radio frequencies of several kilohertz to optical frequencies of several hundred terahertz, resonators are one of the most widely used components. A resonator is a cavity which stores electromagnetic energy by recirculating the power in a closed loop. The energy that a specific resonator stores is highly frequency selective. Resonators are ideally suited to signal processing applications because of this selectivity. By combining numerous resonators appropriately, virtually any signal processing function can be synthesized. Thus, resonators are versatile building blocks.
For optical frequency applications, resonators are fabricated from dielectric cavities. The cavities may have a variety of geometries, such as a disk, a ring, or a straight section of waveguide with reflectors on each end. Ideally, a resonator would be part of an optical integrated circuit (OIC). As such, it would be fabricated on a dielectric substrate by conventional lithographic and etching techniques. A simple yet practical application of an integrated resonator device is shown in FIG. 1.
FIG. 1 is a schematic of a conventional optical track changing device 100 employing a side-coupled resonant cavity. The device 100 consists of a micro-ring resonator 101 side coupled to two optical first 102 and second 103 waveguides, and serves as an optical channel dropping filter. At a specific resonant wavelength, a signal applied at an input port 104 of the first waveguide 102 can be completely diverted to an output or drop port 105 of the second waveguide 103. At other wavelengths, the signal bypasses the ring and exits at a throughput port 106 of the first waveguide.
Such devices have recently been demonstrated as described by Foresi et al., CLE097 conference, paper CPD-22, Baltimore Md., May 1997. Although the devices have not yet reached a practical level of perfection, it is expected that refinements in fabrication will eventually yield useful devices. In general, however, very high quality integrated devices will continue to be a challenge, since the dimensions involved require precision on the order of nanometers or less. These dimensions might be unobtainable with standard processing techniques designed for integrated optics.
On the other hand, discrete resonators (rather than integrated) with qualities approaching the theoretical limit of perfection are now available. These resonators are in the form of micro-particles and micro-spheres, which are relatively inexpensive, and readily available. Micro-spheres, for example, can be fabricated from molten glass, which upon cooling, forms a nearly flawless globe due to surface tension. A quantitative measure of a device's quality is its Q value. The Q value is related to how much energy a device can store. Etched integrated micro-resonators of the sort shown in FIG. 1 have achieved Qs of up to several thousand.
Micro-spheres on the other hand have been reported with Qs of over 10.sup.9. Refer to Gorodetsky et al., "Ultimate Q of optical micro-sphere resonators", Optics Letters, vol. 21, P. 453-455, 1996. Micro-particles have other advantages over integrated resonators. While in a molten state, dopant material may be added to the particle which enhances certain effects. Also, the power circulating in a high Q resonator can build up to extremely high intensities, which is useful in observing nonlinear and quantum effects. Further, micro-particles are discrete, and may be replaced if damaged or if not of the proper dimensions. Several practical devices have been proposed for micro-particle resonators such as filters, switches, micro-spectrometers, and for measuring ultra-small displacements.
To date however, the realization of useful devices utilizing discrete resonators have been restricted by the conventional method of deploying them, which is depicted in FIG. 2. FIG. 2 is a schematic of the conventional method of deploying micro-particle resonators as devices. As shown, a micro-particle 201 is typically placed in proximity to a waveguide or freely transmitted optical beam 202. Light from the adjacent input beam 202 is coupled, as at 203, into the micro-particle where it circulates if the input wavelength is matched to a resonant mode of the particle. Power circulating in the resonator either couples back out into an output beam direction 204 or is evanescently lost to scattering, as at 205, out of the particle or to absorption by intrinsic effects. To observe the greatest effect of the resonant particle on the input beam in the scenario of FIG. 2, all the power on the input beam has to be scattered out of the particle or be absorbed.
The conventional deployment of resonators shown in FIG. 2 is extremely restrictive for a number of reasons. First, in order to scatter all of the input beam power, the interaction between the input beam and the particular resonant mode within the particle has to be precisely controlled. Physically, this means that the placement of the particle with respect to the input beam is critical. Also, different resonant modes of the particle which occur at different frequencies require different optimum placements. Therefore, the device cannot be efficient at all wavelengths. Second, the scattered light might be collected by a photodetector, however, scattering typically occurs in all directions of space, making power collection complicated and inefficient. Third, it is usually desirable to have a device that responds as a so called track changing configuration. That is, an input signal is diverted to a different (but well defined) path or waveguide by the response of the resonator. A track changing device is depicted in FIG. 1, where at resonance the power is completely diverted to the output waveguide.
It is not well appreciated that the micro-particle has a well defined phase response, which except for a scaling factor, is independent of the resonator shape, or its coupling configuration to an external beam. By appropriately using the phase response of a micro-particle, so called ideal track changing response can be achieved. This means that all the advantages of the device in FIG. 1 can be realized without resorting to two waveguides simultaneously coupled to the resonator.